1. Field of the Invention
The present invention relates to integrated circuits and, in particular, relates to miniaturized electrical interconnects having reduced resistance and capacitance.
2. Description of the Related Art
To provide improved performance, manufacturers of integrated circuit devices continually strive to increase circuit density. Such devices are typically formed on a semiconductor substrate, such as a silicon wafer, and comprise a large number of miniaturized circuit elements. These elements, which include transistors, diodes, capacitors, and resistors, are usually disposed within or adjacent the substrate and define a plurality of circuit nodes. To combine the circuit elements into a useful electronic circuit, integrated circuit devices typically include a plurality of conducting paths that link the circuit nodes in a preferred manner. Typically, the conducting paths are provided by electrical interconnects comprising metallic wires including, for example, wires made of aluminum or aluminum alloy that are embedded in an insulating layer such as a layer of insulating SiO2.
However, as circuit density is increased, problems associated with conventional electrical interconnects are becoming more apparent. In particular, a higher density device having an increased number of circuit elements will likely require an even greater increase in the number of electrical interconnects. Consequently, the electrical interconnects will need to have a reduced thickness and adjacent interconnects will need to be spaced more closely together.
Unfortunately, such dimensional reductions tend to increase the resistance of individual interconnects and increase the capacitance between adjacent interconnects, thereby possibly increasing signal propagation delays and signal cross-talk. In particular, electrically charged adjacent conductors acts as the plates of the capacitors. As the distance between adjacent conductors decrease, the resulting capacitance increases. This resulting increase in capacitance slows propagation of signals as the capacitance must be overcome prior to propagation of the signal along the conductor. Hence, while it is desirable to increase device density on integrated circuits, considerations such as these pose problems for maintaining or improving circuit performance.
To improve the conductivity of interconnects, it has been suggested that copper metallurgy be substituted for the aluminum metallurgy that is now typically being used. Advantageously, copper metallurgy interconnects are viewed as having increased conductivity and thus less resistance. The lower resistance of interconnects of this metallurgy could allow the use of smaller dimensions of interconnects thereby facilitating the increase of device density on the integrated circuit. However, several potential problems have been encountered in the development of this proposed metallurgy. One of the main ones being the fast diffusion of copper through both silicon and SiO2. Fast diffusion of copper into silicon or silicon oxide results in diffusion of the conductive interconnect into the surrounding materials which can damage device performance or can result in short circuits occurring between adjacent interconnects.
To decrease capacitive loading, it has been suggested that the interconnects could be embedded in a solid insulating medium other than SiO2, such as a polymer comprising fluorinated polymide. However, as in the case of SiO2, an incompatibility problem with copper metallurgy has been found. In the case of polyimide, and many other polymers, it has been found that the polymer, during the curing, reacts with copper forming a conductive oxide CuO2, which is dispersed within the polymer. This then raises the effective dielectric constant of the polymer and in many cases increases its conductivity. Hence, there have been numerous suggested approaches towards solving the problems of capacitive coupling and increased resistance occurring as a result of a need to formulate smaller dimensioned interconnects that are spaced closer together. A primary difficulty that results is the relative incompatibility of lower resistance materials with the surrounding insulating material.
Silver is one of the best conductors, in that it has the lowest specific resistivity of any metal or alloy. Furthermore, a vacuum is the ultimate dielectric, with air being nearly as good. However, the use of a vacuum introduces additional problems or complexities to the device. The first being the low heat conductivity of the vacuum and the second being the cost of the package required to maintain the vacuum. Air, which has somewhat better thermal conductivity, has its own problems in that both copper and silver react with air to form oxides or other compounds. Alternatively, gold is known to be quite environmentally stable. However it's specific resistivity is higher than that of copper and silver.
To address the problem of increased capacitance, interconnects comprising an air bridge have been developed as described in U.S. Pat. No. 5,891,797. The air bridge is a length of conducting material that extends from a first supported end to a second supported end through an air space such that the bridge is substantially surrounded by air. Consequently, because air has a dielectric constant that is substantially less than that of SiO2, the capacitance between adjacent interconnects is reduced.
However, because the air bridge tends to sag under its own weight, the length of the air bridge is a possible limiting factor. In particular, because the air bridge is only supported at its first and second ends, gravitational forces acting on the air bridge when the bridge is horizontally disposed cause the air bridge to sag such that the unsupported middle of the air bridge is deflected downward with respect to the first and second ends. Because the degree of sagging increases as the length of the bridge is increased, the length of the air bridge cannot exceed that which would cause the air bridge to break or come into contact with another conductor of the device.
According to classical mechanics for simple air bridge structures, the center of the bridge is deflected downward with respect to the supported and constrained ends by an amount δ given by
      δ    =                  ρ        ⁢                                  ⁢                  L          4                            32        ⁢                  h          2                ⁢        E              ,wherein ρ is the mass per unit volume of the air bridge, L is the length of the air bridge, h is the height of the air bridge, and E is the modulus of elasticity of the air bridge. Consequently, aside from the geometric factors L and h, the deflection δ is proportional to the ratio of (ρ/E). Thus, an air bridge formed of a material having a reduced ratio of (ρ/E) will experience less sagging. If the ends of the bridge are not considered to be constrained then
  δ  =                    5        ⁢        ρ        ⁢                                  ⁢                  L          4                            32        ⁢                  h          2                ⁢        E              .  This is the worst case assumption.
ResistivityElastic ModulusMass DensityMaterial(nΩm)(GPa)(mg/m3)ρ/ECopper16.71288.930.0698Silver14.77110.50.148Gold23.57819.30.247Aluminum27.5702.70.039
The table above illustrates the physical properties of possible air bridge materials. Both copper and silver have resistivities that are substantially less than that of aluminum and, thus, would provide air bridges with reduced resistance. Because copper has a ratio of (ρ/E) which is less than that of silver, a low resistance bridge comprising copper would experience less sagging and, thus, would be more suitable for applications that require bridges having extended lengths. Alternatively, because silver has a resistivity less than that of copper, a bridge comprising silver would be more suitable for applications that require reduced resistance. However, as was pointed out previously, both copper and silver are susceptible to environmental degradation in an air environment.
Gold also has a resistivity less than that of aluminum. Furthermore, gold is not susceptible to environmental degradation in an air environment. However, because the resistivity of gold and the ratio of (ρ/E) of gold are substantially larger those of silver or copper, a bridge formed of gold would have a relatively large resistance and would experience a relatively high degree of sagging.
Various processing techniques may also contribute to the effects of device reliability and environmental degradation. For one, annealing is a process involving heating and cooling of a mechanically work-hardened region, which is designed to effect the microstructure of crystalline materials. Annealing typically softens work-hardened microstructures by relieving residual stress caused by mechanical processes, such as polishing and/or grinding. Additionally, for sub-micron structures, chemistry is a substantially important variable for establishing high electrical conductivity in conductive interconnects. The working of mechanical processes may significantly decrease electrical conductivity and retard grain development.
As is known in the art, abnormal grain development may be associated with a duplex grain structure caused by the dissolution of oxides during a high temperature annealing process. The propensity for grain coarsening and duplex grains may be attributed to excessive solution temperatures and oxygen concentrations. Unfortunately, coarse grains formed during high temperature anneals may remain present after cooling. In addition, the rate of cooling from high temperature anneals may also detrimentally influence the mechanical properties of materials comprising high levels of impurities. Furthermore, rapid cooling may also result in substantially high, non-equilibrium levels of impurities in solid solution. Alternatively, slow cooling may allow for the interaction between impurities and oxygen, which may lead to subsequent precipitation from solid solution. Typical high temperature annealing techniques may be considered harmful and may detrimentally effect chemical, electrical, and mechanical properties of crystalline materials, wherein localized inhomogeneities may change with deformation and thermal history, metal purity, and oxygen content.
The reduction in conductor size introduces additional problems as the surface to volume ratio increases, as it must with the reduced conductor size, the specific electromigration resistance decreases. This is a direct result of the fact that the surface diffusion rate is higher than the grain boundary diffusion rate which is higher than the “bulk” rate. As the relative surface area increases the surface diffusion rate, which may be up to two orders of magnitude greater than the bulk rate, becomes more and more significant.
From the foregoing, therefore, it will be appreciated that there is a need for an improved air bridge structure for an integrated circuit that not only provides a relatively small resistance but also is extendable over relatively large distances. It should also be appreciated that there exists a need to improve processing methods associated with air bridge structures for the purpose of increased reliability.